One of the procedures by which circuit points of electronic assemblies are interconnected is called "wire bonding." While wire bonding has other applications, the term is usually associated with the process of interconnecting circuit points of semiconductor devices and integrated circuit devices with fine wires. The process finds very important application in completing connections from surfaces of semiconductor devices and integrated circuit chips to the internal portion of the connector leads which will extend from the packages for those units. The wires used may have diameters in the order of 1 mil or less. They are bonded to circuit points having an area of only a few square mils and the circuit points that are interconnected may be separated by no more than a few mils. Thus it is that wire bonding is conducted on a micro-miniature scale.
In general, wire bonding techniques and apparatus have kept pace with advances in the production of semiconductor and integrated circuit devices so that the cost of producing such devices has decreased greatly. Nonetheless, some difficult wire bonding problems have gone without a satisfactory solution. Among those problems has been difficulty in paying out an adequate length of wire during wire bonding process while leading a wire from one bond to another. Failure to pay out an adequate length of wire, or to properly shape that length, may lead to inadvertent creation of short circuits resulting from the interconnecting wire touching other circuit points. While that problem is generally common to all of the classes of wire bonding, it is particularly troublesome in connection with those wire bonding procedures in which a side of the wire is bonded to the circuit point. In such a case, the wire extends laterally rather than perpendicularly from the circuit point. If the wire connector is not bent away and looped up from the asembly in adequate degree, it may touch other circuit points at points along its length.
In a large proportion of the applications in which wire bonding is used, the bond is made between circuit points that lie in different but parallel planes. By way of example, it is common in integrated circuit and semiconductor production to mount the integrated circuit chip, or the semiconductor dice, upon a platform which also carries the inner end of the external lead structures. Those structures, which often form part of what is called a "frame" are not as thick as is the dice or chip. The interconnecting wire leads from a low elevation at the external conductor to a higher elevation on the chip or dice. The incidence of short circuiting by interconnector wires can be reduced somewhat by beginning at the circuit point on the dice or chip and ending at a second circuit point on the external connector. However, the length of wire that will extend from the first bond to the second is payed out by pulling the wire against the initial bond. It is not uncommon for that bond to break as a result of that pulling. The circuit points, called "pads," on dice and integrated circuits are very often too small to accommodate a second bond in a second attempt to make the required connection. That difficulty is overcome by beginning at the external connector which usually has much larger surface area. But to do that greatly increases the risk of short circuits when side bonding the wires.
The need for side bonding can be avoided by using gold wire and melting the end of the wire to form a ball before making the bond. That solution is not availiable in making bonds with aluminum wire. Melting the aluminum to form a ball results in the production of oxides that prevent proper bonding. Accordingly, it is almost universal practice to bond aluminum wires by laying a side of the wire against the circuit point to which it is to be bonded and then applying sonic energy to the bond surfaces using a sonically vibrating tool.
To prevent that kind of short circuit, as described above, it is required only to provide a length of wire that is greater than the separation of the two circuit points to be interconnected, and to loop that wire up so that it avoids contact with any of the structure in between those two circuit points. That sounds easy, but to accomplish that result has proven to be very difficult. The distances between circuit points and the diameter of the wire is too small to permit the use of pre-cut connector wires. The bonding is completed by bonding one end of the wire to one circuit point, paying out a length of wire sufficient to reach the second circuit point, and then completing the bond at that second point. The wire is taken from a continuous length and is severed after the second bond is completed. In general, it has not been feasible to push on the wire to make available a length of the wire sufficient to complete connection between two points. Instead, wire is payed out after the first bond is made by pulling on it. Pulling is accomplished by moving the first bond away from the source of wire, or, conversely, by pulling the source away from the first bond. In most instances, the standing part of the wire is stored on a spool. However, to provide a means for shaping the wire, it is common to mount the spool with the bonding tool and to extend the wire from the spool through an opening or passageway in the tool. The wire must move through that passageway freely. Relative movement between the first bond and the tool results in the standing part of the wire being moved through the passageway. As the tool is moved from the position of the first bond to the second, the standing part of the wire is payed out by being pulled through that passageway. However, the task of moving the tool includes the requirement to search for the second bonding point. As often as not, the tool is moved past the second bonding point. It must be returned to it in the search procedure. The need for such a return, and the distance that must be traversed in making such a return, is variable. That means that the length of wire that is payed out in the attempt to move from the first to the second bond is variable. That variation in length has been accommodated in the past by the fact that the standing part of the wire is easily moveable through the passageway by which it extends through the tool. If the tool is moved back toward the first circuit point, the tool moves relative to the wire so that the excess length of wire is payed into the passageway rather than payed out of it.
In prior art method, the shape of the wire conductor, as its extends from the first to the second bond, is determined primarily by the stiffness of the wire and the direction in which the tool was moving at the time that it left the first bond. It is not practical to select wire of different stiffness to complete each interconnection because the several pairs of circuit points that must be connected on an integrated circuit chip is different in the case of each of them. Accordingly, in the prior art, the only method available for shaping the loop between the first bond and the second bond has been to control the angle at which the wire is pulled as it leaves the first bond, and to some extent, to control the height to which the tool is lifted as the wire is payed out. That latter expedient is not particularly effective in the case of the prior art method. The tool through which the wire extends is moved upwardly and then laterally over the region of the second circuit point. There, the tool is lowered. If the wire is sufficiently stiff so that it is caused to stand upwardly at the beginning of the loop, then that stiffness will serve to prevent easy bending of the wire at the second circuit point. Instead, the whole wire is moved down horizontally and laid against the pad under the bonding tool, and it extends substantially horizontally away from the pad. While the wire can be made to begin to loop upward at the first bond, the loop characteristic is lost almost entirely in the vicinity of the second bond.